The present invention relates to a method of purifying by-product gypsum by making it into a paste or sludge by means of an aqueous liquid and washing the sludge in a multi-stage washer by the aqueous liquid, whereby a partial stream of waste liquid obtained in the washer is recirculated into the sludge forming stage.
Under the term "by-product gypsum" is to be understood calcium sulfate obtained as by-product in large-scale chemical processing. Such calcium sulfate results from the reaction of calcium compounds with sulfuric acid, whereby the largest amount of the by-product gypsum is yielded in the wet production of phosphoric acid during the decomposition of phosphate ores by sulfuric acid as well as in the production of hydrofluoric acid by chemically attacking calcium fluoride with sulfuric acid. Depending on its origin, the by-product gypsum includes water-soluble impurities such as phosphoric and fluoric compounds which have a detrixental effect for further processing of the gypsum and therefore must be removed.
It is already known how to remove such impurities, by mixing the by-product gypsum with water to make it into a paste, and subsequently by filtering and simultaneously washing the paste with fresh water. An essential disadvantage of this prior-art method is the fact that a relatively large quantity of fresh water is consumed during the treatment, whereby correspondingly large amounts of waste water are accumulated. Waste water obtained in this process contains the impurities dissolved from the raw by-product gypsum. For example, in washing the so-called phosphoric acid gypsum about 0.9 g/l P.sub.2 O.sub.5 and about 0.4 g/l F is contained in the accumulated waste water. Such contaminated water, however, cannot be discharged into channels without corresponding purification. As a consequence, the cost for treating waste water represents a considerable burden in the purifying process of the by-product gypsum and impairs the economy of its production.
Numerous attempts have already been made during the purifying process of the by-product gypsum either to eliminate or to substantially reduce the waste water. For instance, it is known to clean by-product gypsum by its conversion from one hydration stage into another one. It is true that this known method produces no waste water, but it is so expensive that in comparison with the aforedescribed contaminating process of the waste water no effective economic advantage is achieved.
From the German publication DE-OS No. 3,110,829 a method of purifying the so-called phosphoric acid gypsum is known in which for avoiding the accumulation of waste water a part of the waste water is returned into the sludge forming stage and the remainder of the waste water is employed as washing water in the filtering stage for the phosphoric acid. Such a combination of a purifying process with the preceding production process with which the by-product gypsum is obtained, is in many cases undesirable either on the grounds of operational economy or due to spatial limitations, inasmuch as the feasibility of the purifying process depends on the absorption capacity of the preceding production process and in addition, due to this close coupling, the operational equipment for the two processes must be arranged in close proximity to each other.